The invention relates generally to the area of numerical controls, and specifically, the invention provides a method and apparatus for automatically realigning a machine element to a predetermined position after an interruption of power thereto. With most prior art numerically controlled machines, an interruption of power must be followed by a realignment procedure after power is reapplied. In simpler machines, a realignment may be executed by moving the machine element to a predetermined alignment position and establishing values in the numerical control corresponding to this predetermined alignment position. However, on larger profiling milling machines, the realignment is more complex. The machine bed may range from 50 feet to several hundred feet long. Further, the machine bed may have several independently operating multiple spindle gantries associated with it. if predetermined alignment points are used, excessive time is consumed in moving the gantry the length of the bed to these points. Further, since each gantry operates independently, there is a high probability that one gantry would interfere with another gantry's attempt to reach an alignment point. Therefore, the operations of all the gantrys have to be terminated to execute an alignment of one gantry. This, of course, is a very inefficient operation. The disclosed method and apparatus provides for an automatic realignment of the machine elements after the interruption of power thereto.
In some more recent controls, during a power interruption, a nonvolatile memory is used to store a command signal representing the desired position and a current feedback signal representing the actual position of the machine element. When power is reapplied, the command and feedback signals are recalled from memory and identify the exact position of the machine element relative to its desired position.
On smaller machines, the above system may be adequate. However, on larger machines, there are conditions which cause the above system to be inadequate. First, there are several factors which will cause the machine element to move after power is terminated. For example, if a power interruption occurs while the machine element is moving inertia will cause the machine element to continue its motion for a short period. Further, in many drive mechanisms, when power is removed, the mechanism will relax slightly until it reaches a state of unenergized stability. This, of course, results in a small motion of machine element.
Second, many systems have a servo balance offset which produces a constant small fixed error between the desired and actual positions of the machine element. Therefore, as power is interrupted and reapplied, the servo balance offset may change thereby changing the actual position of the machine element relative to its desired position. Consequently, a control system that only stores the desired and actual positions of a machine element at the time of a power interruption would be insensitive to changes in position of the machine element occurring after the power interruption.
The disclosed method and apparatus takes into consideration the above problems and provides a system for any machine which automatically realigns a machine element to its precise position after a power interruption thereto.